Composite materials are used in a wide variety of applications in many different industries due to their high strength and rigidity, low weight, corrosion resistance and other favorable properties. In the aerospace industry, composite materials have become widely used to manufacture aircraft structures and component parts for aircraft structures, such as aircraft ribs, spars, panels, fuselages, wings, wing boxes, fuel tanks, and tail assemblies, because they are lightweight and strong, and therefore provide fuel economy and other benefits. For example, aircraft wing skins and spar webs, and other generally flat components, may be formed of stiffened composite panels comprising panels to which reinforcing stiffeners may be attached or bonded using mechanical attachment means, co-bonding or co-curing techniques to improve the strength, stiffness, buckling resistance and stability of the composite pressure webs or skin panels. Co-bonding generally refers to bonding processes where a pre-cured reinforcing stiffener would be bonded to an un-cured panel. Co-curing generally refers to bonding processes where an un-cured reinforcing stiffener would be bonded to and cured together at the same time with an un-cured panel. Secondary bonding generally refers to bonding processes where a pre-cured reinforcing stiffener would be bonded to a pre-cured panel.
Known reinforcing stiffeners used with such composite webs or skin panels may include C-beam, I-beam or T-beam stiffeners (i.e., beams with C-shaped, I-shaped or T-shaped cross-sections) or other shaped stiffeners, such as hat-shaped or blade stiffeners. However, stiffened composite panels reinforced with such known stiffeners are costly to form and may experience high pull-off loads at the radius filler, i.e., “noodle”, portions of the stiffeners or at the radius common to the attached flange of the reinforcing stiffener in the region of the noodle. As used herein, “pull-off load” means a shear load and/or moment force applied to a composite component part, such as a reinforcing stiffener, at locations where the composite component part is attached or bonded to a structure, such as a composite pressure web or skin panel, such that the shear load and/or moment force may cause delamination or separation of the composite component part from the attached structure. As used herein, “radius filler noodle” means a composite material or adhesive/epoxy material having a generally triangular cross-section that is used to fill a gap left by the radius of curved pieces of a composite component part, such as a reinforcing stiffener.
To decrease the likelihood of delamination or separation of reinforcing stiffeners from composite webs or skin panels due to high pull-off loads, numerous additional radius filler elements, fasteners, and/or angle fittings may be required at the locations or joints where the reinforcing stiffener is attached or bonded to the composite web or skin panel. Such radius filler elements, fasteners, and/or angle fittings may provide additional structural reinforcement to the locations or joints and distribute the shear load and/or moment force in order to reduce the risk of delamination at the radius filler or noodle portions of the reinforcing stiffener. However, use of such numerous additional radius filler elements, fasteners, and/or angle fittings may result in increased production time, increased part count and expense, increased labor and manufacturing costs to install and maintain the parts, and an overall increase in the complexity of the structure. Moreover, the use of fasteners or angle fittings that require mechanical fastening to the reinforcing stiffener or composite web or skin panel may require the formation of appropriately-sized holes in the composite material or structure. This, in turn, may require the use of specialized tooling to form such holes in the composite material or structures. Such specialized tooling may result in further increased labor and manufacturing costs.
Accordingly, there is a need in the art for improved composite stiffeners and improved composite stiffened structures and methods of making the same that provide advantages over known configurations, structures and methods.